This section introduces aspects that may help to facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Radio Units (RUs), including indoors and outdoors (for outdoors, also referred to as Remote Radio Units (RRUs)), are an important part of the radio base station (RBS) system and they are usually fed by a −48V direct current (DC) power system. During the radio traffic, a Power Amplifier (PA) in the RU will generate pulse load which will reflect as current ripple to the power interface of the RU. One example is the time slots in Global System for Mobile Communications (GSM) that can have different output power e.g. full power in time slot #1 and no power in time slot #2 and so on. The result is a square wave shaped current ripple with a frequency of 216 Hz (1on/1off) to 867 Hz (4on/4off). The current ripple depends on a maximum RU power and step load variations on the transmitted radio frequency (RF) signal.
This current ripple will cause both inductive and resistive voltage ripple on the bus-bar due to inductance and resistance of the distribution cables of the power system, as well as the output impedance of the power system. This current ripple and voltage ripple will impact other product's functionality, performance or lifetime, e.g., the system back up battery, since they are connected on the same bus-bar.
The RU is needed to be able to keep functioning when the power interface has less than 10 ms disturbance. To achieve this, a lot of capacitors are connected on the DC input within the RU. Those capacitors will cause high inrush current and need very complicated inrush current limit circuit, which takes a lot of board space and decreases the product reliability.